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wavefront lasik(and prk) increases less HOAs than conventional lasik


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CUSTOMISED PRK using the Zyoptix (Bausch and Lomb) system appears to increase higher order aberrations to a lesser degree than conventional PRK in patients with myopic astigmatism, according to the results of an Italian study presented at the 7th ESCRS Winter Refractive Surgery Meeting.

The study compared 30 patients who underwent PRK with the customised ablation system, with 30 patients undergoing conventional Planoscan PRK. It showed that eyes which underwent Zyoptix ablation had a significantly smaller increase in the root mean square (RMS) of their higher order aberrations at six months follow-up, said Mario Nubile MD.

“With standard Planoscan PRK there was an increase in all aberrations. Coma, trefoil and spherical aberration increased and the total RMS was nearly doubled. With Zyoptix PRK, the coma decreased and there was less increase in trefoil and spherical aberration, which were the main reasons there was less increase in the overall RMS compared to Planoscan,” Dr Nubile said.

In the prospective study the two treatment groups both had myopic astigmatism and a spherical equivalent ranging from -2.5 D to -6.0 D (mean: – 4.6 D). Both groups underwent PRK with the Technolas 217 excimer laser, one group with the wavefront-guided Zyoptix ablation profile and the other with conventional Planoscan ablation.

“Six months postoperatively all eyes in both groups were within 1.0 D of emmetropia and 75% were within 0.5 D. There was no significant difference between the two groups in terms of UCVA and BCVA and both achieved excellent visual acuity. There was a non-significant trend towards better visual acuity in the Zyoptix group,” Dr Nubile said.

He added that several studies have shown that refractive photoablations such as PRK and Lasik increases the overall corneal aberrations and that the increase is dependent on pupil size and ablation depth.

The aberrations which increase most are spherical aberration and coma. Proponents of PRK and Lasek note that these procedures do not induce the flap-induced aberrations seen with Lasik.
The main cause for the reduction in optical quality in laser-based refractive surgery is the modification of corneal asphericity. Other factors include the bio-mechanical response, corneal mechanics and the healing effect, he said.

“An ideal ablation should have different features. It should improve optical quality, induce fewer aberrations and reduce existing aberrations. It should also be aspheric and incorporate mechanical and biological response. However, we are still far from this point,” Dr Nubile said.

The superior results of wavefront-guided ablations may be partly due to the lower amount of tissue that has to be removed - almost 13% less - compared to Planoscan ablations. By removing less tissue, wavefront-guided ablation induces less of a healing response and fewer bio-mechanical changes, he explained.

In support of this hypothesis, Dr Nubile pointed out that among those in the Zyoptix group, the higher the preoperative higher order RMS value, the lower was the percentage increase of aberrations.
Conversely, among those with low levels of preoperative higher order aberration the percentage change was similar, whether they had undergone Zyoptix or Planoscan ablation.

Dr Nubile suggested that in such eyes the healing response even to customised ablation is still enough to negate any potential reduction of higher order aberrations.
Thomas Kohnen MD, who chaired the session at the Rome meeting, noted that he had observed the same trend in patients treated with wavefront-guided Lasik.

“When we treated with customised Lasik we found that we sometimes even increased the higher order aberrations in patients with low amounts of higher order aberrations preoperatively, while we maintained it among those we treated with high amounts of higher order aberrations,” Dr Kohnen said.

Dr Kohnen said that this raised the question of whether patients with only small amounts of higher order aberrations should undergo customised ablations.
In response Dr Nubile pointed out that in his study patients with low amounts of higher order aberrations undergoing Zyoptix ablations did as well as similar patients undergoing Planoscan ablations.

Therefore, his results did not indicate that customised ablations posed any hazard to such eyes. Furthermore, the majority of patients have enough aberrations to potentially benefit from the wavefront-guided approach, he said.

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Re: wavefront lasik(and prk) increases less HOAs than conventional lasik


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The Trouble with Wavefront
The improved outcomes seen with custom ablation may not be attributable to this technology at all.
Christopher Kent, Senior Editor
 
IT'S EASY TO BELIEVE THAT WAVEfront-guided ablation is superior to standard ablation. After all, today's wavefront-guided procedures have consistently produced better outcomes than traditional procedures--at least in the United States.

According to Jack Holladay, MD, MSEE, FACS, clinical professor of ophthalmology at Baylor College in Houston, however, the improvement in outcomes may be the result of three advances in technology that have nothing to do with customized wavefront. In fact, he says, these same advances can be made part of standard software at a much lower cost to doctor and patient, producing outcomes at least as good. (Dr. Holladay says this has already happened internationally, especially in Europe.)

"Three changes have been made to the software in wavefront-guided systems that haven't been made to standard system software, at least in the United States," he says. "As a result, wavefront-guided ablations do three things standard ablations do not: They keep the cornea prolate, produce larger astigmatic corrections, and reduce the formation of central islands. I believe these three changes account for the improvement in performance seen in comparative studies."

In fact, Dr. Holladay says that the whole idea of correcting higher-order aberrations by ablating the cornea is fundamentally flawed. "The Food and Drug Administration won't allow manufacturers to claim that these systems correct higher-order aberrations, because manufacturer's studies for FDA approval haven't provided any scientific evidence that they do."

 
Jack T. Holladay, MD, MSEE, FACS
 
Houston surgeon Jack Holladay cites this example in which the wavefront treatment area is almost 25 percent larger than the standard treatment area based on the same prescription.

Keeping the Cornea Prolate

Laser ablations tend to produce an oblate cornea, says Dr. Holladay, because many of these lasers were first calibrated using a flat surface, not a dome-shaped surface like the cornea. "Any electromagnetic energy striking a surface, including excimer laser energy, becomes more spread out and less effective when the surface is tilted relative to the light source," he notes. "The laser's effective energy drops off as the beam moves away from the center of the cornea, by 16 percent at 1 mm; 20 percent at 2 mm; and 28 percent at 3 mm. [See graphic, page 65.] This is the reason these lasers produce an oblate cornea; they're not ablating as effectively away from the center of the cornea. All that's necessary to correct this is to embed an energy compensation curve into the software."

Dr. Holladay says correcting this problem is important for two reasons. First, making the cornea oblate increases spherical aberration. "The surface of the cornea in front of the scotopic pupil must be prolate in order for the rays in the periphery not to bend too strongly, causing blurred focus," he says. "Making the cornea oblate increases the total spherical aberration of the eye, producing nighttime halos and glare complaints."

Second, an oblate ablation shrinks the optical zone. Three years ago, Dr. Holladay demonstrated that if you make the cornea more oblate, a greater optical correction produces a smaller functional optical zone.1 "For example," he says, "we did two -12 D treatments using the LaserSight laser. Our intention was to create a [sign in to see URL] optical zone. A standard treatment with the resulting oblate cornea produced an effective [sign in to see URL] optical zone. By doing nothing different for the second patient except compensating for the drop-off in energy toward the periphery, we produced an [sign in to see URL] optical zone." He has shown that a -5 D treatment without prolate compensation shrinks the optical zone by about 10 percent; a -10 D treatment shrinks it 25 percent.

Dr. Holladay adds that any excimer laser system can upgrade its software to make this correction and produce a prolate cornea. "With one exception—Wavelight's Allegretto—laser manufacturers haven't added this feature to their standard laser software in this country, only to their wavefront-guided software," he says. "Not surprisingly, the WaveLight Allegretto gets the best results of any standard laser ablation system in America today. Its results are equivalent to the wavefront-guided treatments of Visx, LADARVision and Technolas." 2 He notes that this feature is part of the standard software in Europe, and says that surgeons he has spoken with in Europe are favoring topography-customized ablations or standard prolate software instead of wavefront-guided systems.

"This prolate compensation is the major reason for the improved outcomes seen with wavefront-guided systems," he adds, "not customized wavefront measurements."

Sizing with the Smaller Axis

The second significant change made in wavefront-guided laser software, according to Dr. Holladay, involves the size of the astigmatic ablation zone. An astigmatic correction takes the form of an elliptical or oval shape which can be defined by two axes, one short (minor) and one long (major). Previously, when a laser was set to create a 6-mm optical zone, the software produced an astigmatic correction with a major axis that was 6 mm wide. This made the short axis less than 6 mm wide.

Dr. Holladay says a standard plano [sign in to see URL] x 90 degree treatment set for a 6-mm optical zone actually ends up being 4.5 by 6.0 mm. "About three years ago I explained to the FDA that when a 6.0 mm optical zone is specified it must be at least 6.0 mm in both the major and minor axes to be correct and avoid potential halos at night," he says. "They agreed, and as a result a plano [sign in to see URL] x 90 degree wavefront ablation is now 6.0 by 7.5 mm. This makes the wavefront treatment area almost 25 percent larger than the standard treatment area based on the same prescription.

"This means that when we compare overall results involving astigmatic correction done by wavefront-guided systems with results achieved using traditional system software, we're comparing different size optical zones that are labeled as being the same! Is that a fair comparison? Absolutely not." He notes that a substantial number of patients in those comparative studies had astigmatism of 3 to 4 D, more than enough to have a profound impact on the resulting quality of vision.

Cleaning Up the Islands

The third change made in wavefront-guided systems is improved software designed to reduce islands in the center of the treated zone. Dr. Holladay says the specific change is proprietary and hasn't been published by any of the companies, but like the other two improvements, in America this has only been incorporated into the software used in the wavefront-guided lasers.

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Re: wavefront lasik(and prk) increases less HOAs than conventional lasik


He notes that several European laser systems currently provide all three upgrades as part of their standard laser package, including Carl Zeiss Meditec, Mel 80, Schwind and Wavelight. "However," he adds, "only Wavelight was willing to spend the time and money to get FDA approval to add these three features to its traditional laser system software in the United States."

A Problematic Premise

In spite of the improvement caused by the addition of these features, custom wavefront guidance leads to ablations that often increase higher-order aberrations, Dr. Holladay says. The exception: spherical aberration, which isn't increased because the wavefront-guided systems keep the cornea prolate.

The reason, according to Dr. Holladay, is that most of the higher-order aberrations (excluding spherical aberration) are located in the crystalline lens; ablating the cornea won't address that. To correct this is problematic if four ways:
Correcting higher-order aberrations on the cornea creates an on-axis system. "The eye is a two-lens system," says Dr. Holladay. "Correcting a tiny bump on the crystalline lens with a tiny ablated divot on the cornea can only cause limited on-axis improvement for a single point at distance, such as a star. Points that are just a little off-axis may actually be made worse. If you're looking at a printed word, the center letter will be clear, but the letters further away will get progressively less clear.


 
Surface tilt undercuts the impact of laser energy away from the center.

"Most surgeons know intuitively that it's inappropriate to correct problems on one surface by changing another surface," he adds. "That's why many surgeons are moving to refractive lensectomy when the problem is in the crystalline lens."

Changing the cornea changes the path of the light rays. Dr. Holladay notes that making a change in the cornea to correct a problem in the crystalline lens assumes that the "corrected" light rays will follow the same path through the crystalline lens. In fact, he says, changing the cornea assures that the light will take a different path. "The crystalline lens isn't symmetrical; it's aspheric with a gradient index of refraction," he says. "So, the 'correction' can't be effective, and it may even make vision worse. This could account for some of the increase in higher-order aberration noted after customized wavefront treatment." In addition, Dr. Holladay observes that even if you could line up the correction and the flaw perfectly, the alignment wouldn't last; the optics of the crystalline lens change as we age.

Laser systems don't compensate for pupil/cornea misalignment. "The average person's pupil is nasally displaced about 0.4 mm from the geometric center of the cornea," Dr. Holladay points out, "putting the line of sight about 5 degrees away from the optical axis of the eye. So, when we center the laser over the pupil, we're centering the treatment over the nasal side of the cornea, not the top of the cornea. Unless the software compensates for this, we end up treating asymmetrically, inducing coma and irregular astigmatism."


Releasing the stretch on the LASIK flap induces aberration. Dr. Holladay notes that when tissue is stretched, irregularities are smoothed out. This is why a fully inflated rubber balloon is shiny, producing a clear reflection, but the surface loses its optical smoothness and doesn't produce a clear reflected image as it deflates. He notes that the same principle applies to corneal tissue. "Before a LASIK flap is cut, the corneal collagen fibers are under tension created by intraocular pressure. Once we cut the flap, the corneal fibers in the flap are no longer on stretch and optical smoothness is reduced."
The Topography-Guided Alternative

Because higher-order aberrations are primarily in the crystalline lens, Dr. Holladay says the only truly effective way to correct them is to treat the aberrations directly with surgery or replacement of the crystalline lens. However, he acknowledges that altering the cornea can certainly improve vision. "Treatments such as conductive keratoplasty or Intacs often exaggerate the prolate shape of the cornea, compensating for positive spherical aberration in the aging lens," he notes.

"Also, it is possible for the cornea to be the source of some higher-order aberration besides spherical aberration. When that's the case, correcting it by altering the corneal surface makes sense, but topography is much more precise for this measurement than wavefront." He adds that many doctors he has spoken to outside of the United States are foregoing custom wavefront and using custom topography-guided systems to correct the problems located on the surface of the cornea, with excellent results.

"I believe that in the future wavefront measurement will still be used to determine where problems are located and how much spherical aberration needs correction. But we'll use asymmetrical, topography-guided treatments instead of wavefront-guided, and we'll get better results."

From Myth to Reality

Given Dr. Holladay's observations, it seems clear that the only way to judge the true capabilities of custom wavefront is to conduct a comparison between a custom wavefront system and a traditional system that share the same software advantages.

"So far, wavefront is much like 'The Emperor's New Clothes' ... a story with no real substance," he says. "Hopefully, the success of Wavelight's Allegretto system with its improved standard software, and a clearer understanding in the United States of what's actually happening, will pressure manufacturers to add the three upgrades to the standard software in all lasers. This will relieve our patients and practices of an unnecessary financial burden and wasted time making unnecessary measurements. It may also force the creative minds in the industry to find ways to really make wavefront-guided ablation do what we've always hoped ... reduce unwanted higher order aberrations. The result of that revolution could be a truly impressive increase in our ability to restore or even improve the quality of vision." RO





Wavefront adds an automatic measurement of more subtle distortions (called higher order aberrations) than just nearsightedness, farsightedness, and astigmatism corrected by conventional LASIK. However, these “higher order aberrations” account for only a small amount (probably no more than 10%) of the total refractive error of the average person’s eye. Conventional LASIK increases higher order aberrations. Although wavefront-guided treatments attempt to eliminate higher order aberrations, results from the clinical studies have shown that the average aberrations still increase, but less than they do after conventional LASIK. In a few studies comparing wavefront-guided LASIK to conventional LASIK, a slightly larger percentage of subjects treated with wavefront LASIK achieved 20/20 vision without glasses or contact lenses compared to subjects treated with conventional LASIK.

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Re: wavefront lasik(and prk) increases less HOAs than conventional lasik



long article pointing out the risks

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Re: wavefront lasik(and prk) increases less HOAs than conventional lasik


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His abberations increased with lasik then he was moderately abberated and lost one line of vision. Wavefront removed some of the induced abberations and restored his 20/20 and improved night vision. He still had less abberations before lasik but the slight increase in those abberations probably didnt negetivately impact his vision too much. Some people are less fortunate, especially those who had few abberations to begin with
11/18/2005, 8:18 am Link to this post Send Email to Myope5   Send PM to Myope5
 
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Re: wavefront lasik(and prk) increases less HOAs than conventional lasik


misleading testing of visual aspects, even visual accuracy.


When I got my lasik evaluation at my local center, a 15 min drive from me, I had a manual refraction and one done by a refractometer. I was straining so hard to try to make out the 20/25 line that I got "overcorrected" by a full diopter in each eye! I was like this cant be, I am not -6.5 and -6! She says I guess you must have been straining and it picked up this.

I also got a manual refraction and said "I think theres an o somewhere" and "I think the letter to the left is an L" and he said good! He marked my BCVA as 20/20 to 20/25 but theres NO way I can see any of the 20/20, he told me try so I guessed. There was a good chance of an O "somewhere" on the 20/20 line. I probably guessed half of the 20/25 line. I wonder if this is what he writes down for those post lasik!

I have read before that they try to inflate 20/25 or better results as much as they can. If the patient reads or guesses(whatever) even two or three of that line, hes considered 20/xx whatever line he read/guessed and got "lucky" He considered me at least 20/25 but I cant really make them out, just too small without guesswork. I think its safe to deduct half or even a full line from what the post lasik person saw. If he saw 20/25 and half of the 20/20 I consider him 20/25. If he has a hard time seeing 20/25 but is considered 20/25 hes more likley a 20/30. Then theres the whole issue of vision quality which snellen charts arent the best indicator of. Even if he saw 20/20, is it sharp and black or blurry, ghosted, grayish and lacking contrast? How accurate was he at 20/20?

One guy who got lasik learned the hard way the difference between his true 20/20 with glasses and the diminished quality "lasik" 20/20. That line was much harder to see and of poorer quality. Even the 20/25 line wasnt as sharp. I would probably consider him a 20/25 at best and perhaps a 20/30. I can see the 20/30 line sharp, less so in right eye.
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high order abberations
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When comparing wavefront to standard LASIK there was an attempt during an Ophthalmic devices panel meeting to include the word 'reducing' when referring to higher order aberrations. Aberrations induced by wavefront are reduced when compared to standard LASIK, but they are CERTAINLY not reduced when compared to the virgin eye. Wavefront treatments, in fact, were found to induce aberrations in virgin eyes in clinical trials. A conscientious doctor on the panel put the skids under the use of the term 'reducing' when describing wavefront aberrations. Thank you, Dr. Bullimore.
-----------------------------------------------
Excerpt:
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DR. WEISS: Can you repeat the first one again?

DR. GRIMMETT: Sure. Wavefront-guided LASIK does not reduce the level of higher-order aberrations of the preoperative eye.

DR. WEISS: Would that not be confusing to someone? Wouldn't that be confusing?

DR. GRIMMETT: Michael Grimmett.
It may suggest somehow wording in that wasn't it that the higher-order aberrations were 20 percent higher than the preop eye in the wavefront-guided versus what, 80 percent was the number?

PARTICIPANT: Seventy-seven percent.

DR. WEISS: In here, is there any place saying that LASIK itself increases aberrations and that customized corneal ablation increases them less than conventional treatment?

DR. GRIMMETT: I think that's the idea.

DR. WEISS: So maybe we could put that wavefront-guided ablation ??

DR. GRIMMETT: Conventional LADARVision LASIK increases higher-order aberrations by that figure 77 percent while wavefront-guided LASIK increases them by whatever, 20 percent, whatever the number is, or you can say reduces them to a 20-percent level, if you want to use the word "reduces."

DR. BULLIMORE: I would avoid the term "reducing."
-------------------------------------------------
So when you say spherical aberration is 'reduced', it's reduced compared to WHAT, in what population? Is spherical aberration 'reduced' for large pupil patients as well (likely NOT). Are total higher order aberrations decreased?

Please keep in mind that in VISX clinical trials, from the data we have available... 1 in 5 patients did not hit 20/20 at the 12 month mark.

So wavefront can't even do reliably what glasses can do. Let alone the induced corneal distortions and fried nerves.

I completely agree with Dr. G on this. The overwhelming liklihood is that refractive surgeons are still out there giving patients corneal distortions without their informed consent.

I think patients who do their research will find that the strongest, healthiest and least aberrated corneas they will ever have are their virgin corneas.

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How impressed are you all that have had PRK with your vision quality post PRK (after several months of stabilzation and improvement)? How noticeable and annoying are the visual aberrations that come with PRK?

I just want to get some qualititive observations from those that have had PRK. Maybe they will get me off the fence regarding PRK. I still would consider pretty hard to convince. Here's why:

Despite what many claim they experience, you cannot have better vision post PRK than you would have pre-PRK with correction. All types of refractive surgery induce higher order aberrations. Even customized wavefront ablations. The aberrations that wavefront guided ablations induce are just not as severe.

From the Bausch and Lomb website regarding their Zypotix vision correction systems which couples their Zywave aberrometer and their orbscan topographer with their Technolas 217z laser:

Although the ZyWave® Wavefront System measures the refractive error and wavefront aberrations of the human eyes, including myopia, hyperopia, astigmatism, coma, spherical aberration, trefoil, and other higher order aberrations through fifth order, in the clinical study for this PMA, the average higher order aberration did not decrease after Zyoptix Personalized Vision Correction.
The aberrations induced by wavefront PRK are simply not as bad as those induced by traditional PRK. What does this mean?: All refractive surgeries induce aberrations, which can lead to decreased contrast sensitivity (among other things such as haloes, double vision, starbursts), and it is impossible, at least with todays technology, to have better vision with PRK than what you would be able to get with contacts. This is especially true with RGPs, which can decrease higher order aberrations by around 70% (much better than you would get with PRK, which at best is no net gain in aberrations).

You definetely get a reduction in the quality of your vision with PRK. However I can see why the military has been such an enthusiastic adopter of PRK once it was proven as medically safe: It's good enough vision to get the job done (pilots included) and a much preferred alternative to glasses and contacts which can problematic in a combat zone. It's the issue between quantity (Snellen acuity) and quality (how good is you vision, even if it is 20/20).

All these folks who tout how great their vision was after PRK, I think don't remember how good their vision was corrected before PRK, especially with contacts. Especially with bilateral PRK, which is most common right now, you have no reference point to judge you post PRK vision with. With me, for example, if I go without glasses or contacts for a few days, I will oftentimes say to myself, "Hey, my vision isn't so bad...". However the minute I put my contacts in, it's "WOW! What a difference!"

I am hoping that the Coast Guard will adopt the Navy's new SNA contact lens policy. Granted, it usually takes the Coast Guard a few years before they will adopt Navy policy, so I probably shouldn't be too optomistic that the Coast Guard will have this policy in the books in time for my second tour about 3-3 1/2 years from now.
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Typically everthing I have heard about PRK is great and everybody seems like they are 20/15 or atleast 20/20. I guess there is always the exception, which nobody really wants to hear about. I had PRK August 2002, after about 9 months I was about 20/30, but I had lost my best corrected vision. I was no longer correctable to 20/20 for some [sign in to see URL] had a touchup procedure May 2003, and now over 1 year later I am pretty much 20/30, but only correctable to 20/25. Not really sure what to do at this point, but I certainly can't apply because you must be 20/20 correctable. Not necessarily a bad experience, but it has been a trying one.


*this just goes to show you have to push the hype aside and acknowlege the risks! He got induced aberrations and lost 1 line of BCVA. Its true a number of people end 20/20 but this is far from guaranteed
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